1. Field of the Invention
The present invention is broadly concerned with an improved extrusion cooking device and method wherein the extruder is of minimal length to reduce equipment and maintenance costs. More particularly, the invention pertains to such a device wherein the internal bore of the extruder barrel is of tapered, generally frustoconical configuration and the extruder screw is correspondingly tapered. In the production of expanded feed products, the extruder preferably includes means presenting a material flow restriction intermediate the inlet and extrusion die. Where dense, fully cooked, low moisture sterilized feeds are desired, the extruder is operated without a mid-barrel flow restriction, and the extruder is operated to generate high pressure within the barrel immediately adjacent the extruder die. Extrusion devices in accordance with the invention are operated at high speed and can essentially match the throughputs and product qualities of much larger machines.
2. Description of the Prior Art
Extrusion cooking devices have long been used in the manufacture of a wide variety of edible and other products such as human and animal feeds. Generally speaking, these types of extruders include an elongated barrel together with one or more internal, helically flighted, axially rotatable extrusion screws therein. The outlet of the extruder barrel is equipped with an apertured extrusion die. In use, a material to be processed is passed into and through the extruder barrel and is subjected to increasing levels of temperature, pressure and shear. As the material emerges from the extruder die, it is fully cooked and shaped and may typically be subdivided using a rotating knife assembly. Conventional extruders of this type are shown in U.S. Pat. Nos. 4,763,569, 4,118,164 and 3,117,006.
Most conventional modem-day extrusion cookers are made up of a series of interconnected tubular barrel heads or sections with the internal flighted screw(s) also being sectionalized and mounted on powered, rotatable shaft(s). In order to achieve the desired level of cook, it has been thought necessary to provide relatively long barrels and associated screws. Thus, many high-output pet food machines may have five to eight barrel sections and have a length of from about 10 to 20 times the screw diameter. As can be appreciated, such long extruders are expensive and moreover present problems associated with properly supporting the extrusion screw(s) within the barrel. However, prior attempts at using relatively short extruders have not met with success, and have been plagued with problems of insufficient cook and/or relatively low yields.
In recent years, attempts have been made to use extrusion equipment in the fabrication of pelleted feeds. Extrusion is desirable in this context because extrusion conditions effectively sterilize the products. However, pellets produced by traditional extrusion methods are often too hard and do not dissolve readily in water. Such hard pellets may pass through the stomach of monogastric animals with the pellets remaining largely intact and non-digested. Another problem associated with extrusion-produced feeds is that nutrients such as amino acids and vitamins may be substantially degraded and heat-denatured during processing. On the other hand, products produced using conventional pellet mills, though having many desirable physical and nutritional properties, are insufficiently heat processed and cooked so that harmful bacteria may remain in the pelleted products. In response to these problems, it has been suggested to employ a dual component apparatus in the form of an extruder (sometimes referred to as an "expander") which is coupled to a pellet mill. The starting materials are thus cooked in the extruder section, and ultimately formed in the attached pellet mill. This dual component apparatus is relatively expensive however, particularly for the production of animal feeds.
There is accordingly a need in the art for improved, low-cost, short length extruder devices which are essentially equal with conventional long-barrel extruders in terms of product throughput and quality. In addition, there is a need for an extruder apparatus which can produce feeds containing substantially non-degraded nutrients which are highly cooked and have desirable digestion properties similar to those of traditional feeds produced using a pellet mill.